@deshipu WindowMaker is just a window manager, not a platform (unlike GNOME and KDE, and, well, GNUstep), though it itself uses a NeXT-style UI.
Again, GTK2 itself is not a platform; it's a toolkit. A toolkit is a part of the platform, the most important part, perhaps, but still. There is system UI, there are UI guidelines (HIG). MATE, GNOME and Elementary all use gtk3, and yet look at how different they are. NeXTSTEP and Mac OS X (Aqua) use Cocoa and yet look nothing alike.